The energy storage
problem

a briefing document

Energy
storage is part of a series
of briefing documents on the problems of power consumption,
posed by the steady depletion of fossil fuels and most particularly
of pumpable oil.One of a
grouping of documents on global concerns at abelard.org.

The
storage problem

If energy is not wanted immediately,
for instance to switch on a light, some means of storage of energy
required for later use has to be achieved. A battery, a dam, a
gallon of petrol, a hydrogen fuel cell, a log for the fire, or
radioactive sources, are all means of storing power/energy.

Learn
to think clearly about the difference between generating and storing
power.

A power station, or a growing tree, are means
of generating usable power.

A log is a store of energy,
but a power station is not. The power station uses means such
as oil or uranium to store energy prior to using it in the generating
process.

For completeness, an engine is a device that
converts energy from one form to another. Thus, a car engine converts
petrol into moving along the road, while a tree converts sunlight
into logs.

Even insulation your house and increasing
efficiency of processes, thus countering waste, has similarities
to storage.

All conversion to energy involves inefficiencies.
Energy may be converted to storage or to produce work. Thus, energy
is involved in manufacturing a battery or fuel cell, energy is
used to charge up the battery. After that, the energy in the battery
may be used to drive a car, or to run a laptop computer.

Oil,
in the form of gas (petrol) or diesel, may be used to run a car,
or the oil may be used to drive electricity generators in a power
station. The electricity may then used to charge a battery, and
the battery then may be used to drive a car. At
each stage, energy is lost through inefficiencies. These inefficiencies contribute to making storage a bottle-neck issue in cleaning and
modernising energy production.

molten
salt storage

“SolarReserve will build power plants designed as Solar Power Towers. This configuration
captures and focuses the sun's thermal energy with thousands of
tracking mirrors (called heliostats) in a two square mile field.
A tower resides in the center of the heliostat field. The heliostats
focus concentrated sunlight on a receiver which sits on top of
the tower. Within the receiver, the concentrated sunlight heats
molten salt to over 1000 degrees Fahrenheit. The heated molten
salt then flows into a thermal storage tank where it is stored,
maintaining 98% thermal efficiency, and eventually pumped to a
steam generator. The steam drives a standard turbine to generate
electricity.”

Area of heliostats [mirror] field: approximately
two square miles or 1,200 acres

Height of power tower:
approx. 600 feet

Power generated: between 100-600 megawatts,
depending on configuration.
[1MWatt supplies approximately
1,000 U.S. households.]

Molten salt: mixture of sodium
and potassium nitrate. (When solid, this mixture is used as garden
fertiliser.)

Salt energy storage
is more efficient with tower solar units than with parabolic units.

Length of parabolic mirror with oil-filled
tube (purple in sun ray diagram).
In this photo the mirror
is facing the ground for inspection, and is reflecting the ground
and people.
photo credit: Michael
Kanellos/CNET News.com

With
parabolic mirrors, molten salt is run through the tube, instead
of oil. Thus, there are much longer runs of piping over a whole
field of parabolic mirrors to be heated, rather than the sun’s
heat being directed on a central tower and there absorbed by the
molten salt. So, for parabolic mirror systems, more piping, molten
salt and insulation is required, making this storage method more
applicable to the tower configuration.

fuel
formation

Hydrogen and methanol are fuels created as a
means of storing energy effectively.

As well as producing
hydrogen by electrolysis, the gas can be made by high-temperature
disassociation. The heat generated by a solar power plant could
be used for making hydrogen, as well as for driving steam turbines
or, perhaps, space heating.

battery
and fuel cell storage

pumped
storage

With pumped storage, water is pumped
to the top of, say, a mountain during off-peak hours, then dropped
during peak hours to turn turbines, and so generate electricity.(The
turbines that generate the electricity are used in reverse to
pump the water back up the feed pipe.) This generation method
is very expensive.
Examples: Dinorwic/Dinorwig
power station, Llanberis, Snowdonia, North Wales;

other

Other,
more esoteric methods include flywheels (also used for smoothing
power output), capacitors (including super-capacitors - experimental),
and even underground gas compression is under sporadic discussion.